AVULSIONS AND ANCIENT SETTLEMENT PATTERNS IN LOWER MESOPOTAMIA

Lower Mesopotamia can be considered a unique human-environmental interactive system, where society was totally dependent on river waters. Ancient settlements are closely associated with abandoned river courses. The low-gradient Tigris-Euphrates deltaic plain with high aggradation rates and raised alluvial ridges was a favorable site for avulsions during Holocene. Postglacial sea-level change, migration of the Persian Gulf shoreline, changes in floodplain aggradation rates, climate fluctuations and tectonic movements of Mesopotamian depression are suggested as possible geological controls of avulsions. Periodic high floods and tendency for increased in-channel sediment deposition were important in triggering avulsions. The overall effects of avulsions on settlement patterns and population migrations were likely complex and depended on the avulsion styles (progradational or reoccupational), channel patterns (multiple anastomosed or single-channel), avulsion frequencies and rates of abandonment. Three distinct types of channel patterns that could significantly influence settlement type (rural or urban), size and distribution were identified: 1) single meandering channels with crevasse splays, 2) anastomosed channel networks formed by progradational avulsions with natural lifetime of about 100 yrs, 3) long-lasting (10²-10³ yrs) systems of several active channels formed by frequent avulsions and/or slow abandonment. Large-scale human activity (e.g. canal, dike, dam construction, channel maintenance, and flood control) could significantly alter natural avulsion processes.